Ontwsep™ SAX/PSA SPE CartridgesOntwsep SAX PSA has equal amounts of SAX (upper layer) and PSA (lower layer), separated by a polyethylene sieve plate in the centre. PSA is an Npropylethylenediamine bonded silica gel, whereas SAX is a quaternary ammonium salt bonded silica gel. Both materials are designed to remove acidic interferences such as fatty acids and organic acids and are often used to detect pesticides in food.